The present invention generally relates to recuperators for turbine engines and, more particularly, to an apparatus and method of catalytically treating within or immediately about a recuperator an exhaust gas from a turbine over a broad operating range of engine loads and ambient conditions.
Gas turbine engines commonly employ a compressor for compressing air and a combustor for combusting compressed air and fuel. Hot exhaust gases from the combustor are fed to a turbine to drive a driveshaft. Turbine exhaust is fed to a recuperator that places the exhaust in heat exchange relationship with compressed air from the compressor. The heat exchange heats the compressed air and thereby enables heat recovery by the heated compressed air flowing to the combustor. The result is improved engine efficiency.
However, in a gas turbine engine, exhaust gases typically contain undesired emissions such as carbon monoxide and unburned hydrocarbons. To ameliorate the deleterious effects of these undesired emissions before they are discharged into the atmosphere, the emissions can be reduced through catalytic oxidation and/or reduction. The oxidation and reduction reactions produce relatively harmless products such as carbon dioxide and water.
Carbon monoxide is produced in the gas phase-oxidation of fuel in the combustor of the gas turbine engine, including combustors that contain catalysts (such as in U.S. Pat. No. 5,685,156) and thereby allow them to operate very lean and create very little NOx. In this case, the CO produced in the gas phase combustion may be further oxidized to CO2 when it contacts the surface of the catalyst. However, there is typically only enough catalyst to burn fuel to develop surface temperatures high enough to initiate the gas phase reaction. Consequently, only a portion of the CO diffuses to the surface of the catalyst from the gas stream before the gas exits the catalyst section. The residual CO remains in the gas and causes the CO emissions from the gas turbine to remain unacceptably high.
By way of a regenerator, as distinguished from a recuperator, catalytic removal of constituents is sought in U.S. Pat. No. 3,641,763. A regenerator is provided that employs a rotating matrix that is subjected to both exhaust gas from a turbine and cold compressed air from a compressor. The matrix is coated or impregnated with a catalyst. Thereby, the rotating matrix is intended to accomplish both heat exchange and catalysis. Similarly, and in the context of a turbocharged reciprocating engine, U.S. Pat. No. 3,713,294 provides a regenerator impregnated with a catalyst.
While the above designs address, to a limited extent, catalysis by a regenerator, they do not address catalysis by means of a recuperator. Further, a method of placing or coating a catalyst immediately upstream of or within a recuperator is evidently unaddressed by the prior art. In particular, the prior art does not appear to address optimizing system efficiency by selectively placing a catalyst within or upstream of the recuperator. Also seemingly omitted from the prior art is a method of reducing NOx emissions by tuning the combustor to produce lower NOx levels at the turbine exhaust and catalyzing the recuperator to ameliorate the CO levels that are consequently increased.
Placement of a catalyst upstream of or within the recuperator puts the catalyst in an operating environment that is optimal for both performance and life. Placement within the combustor (as in U.S. Pat. No. 5,685,156) results in operating temperatures that are too high for long life with conventional, state-of-the-art catalysts. Placement downstream of the recuperator, i.e., in the engine exhaust duct, as is common in many applications, results in temperatures that are too low for efficient catalysis, particularly at low engine loads or in cold ambient conditions.
As can be seen, there is a need for an apparatus and method of catalyzing undesired constituents by means of a recuperator and/or via a method of placing or coating a catalyst within or immediately upstream of a recuperator. Another need is for a method of tuning a turbine, such as a microturbine, to lower NOx levels while ameliorating CO levels that would otherwise be increased with lower NOx levels.
Accordingly, in one aspect of the present invention, a fixed boundary recuperator comprises a gas inlet through which flows a gas; a gas outlet in communication with the gas inlet; an air inlet through which flows air, with the air being in heat exchange relationship with the gas; an air outlet in communication with the air inlet; and a catalyst disposed at least at one of a plurality of locations, with the locations selected from the group including a position immediately upstream of the recuperator and a position within the recuperator.
In another aspect of the present invention, a method of controlling undesired emissions from an engine comprises selecting a desired NOx emission level; altering at least one of a fuel-to-air mixture and an operating temperature in a combustor to produce the desired NOx emission level in a combusted gas having the undesired emissions; flowing the combusted gas into a recuperator downstream of a turbine; and catalyzing the undesired emissions at least at one of a plurality of locations, with the locations selected from the group that includes a position immediately upstream of the recuperator and a position within the recuperator.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.